In recent years, with miniaturization and reduction in the weights of electronic devices such as mobile phones and laptops, an increase in the capacity of secondary batteries, which are power sources of these electronic devices, has been increasingly demanded. For example, nonaqueous electrolyte secondary batteries (e.g., lithium-ion secondary batteries) meet the demand.
In general, an electrode group, which is formed by winding or stacking a positive electrode and a negative electrode with a separator interposed therebetween, is used in a nonaqueous electrolyte secondary battery.
Patent Document 1 shows a nonaqueous electrolyte secondary battery using a wound electrode group. In the battery, the filling amount of an active material formed on the inner circumference of a current collector is smaller than the filling amount of an active material formed on the outer circumference of the current collector, and the ratio of the filling amount of the active material on the outer circumference to the filling amount of the active material on the inner circumference gradually increases from the center of the wound electrode group toward the outside in order to improve the cycle characteristics of the battery. This reduces the difference between the filling amount of the active material on the inner circumference and the filling amount of the active material on the outer circumference in the entire region of the wound electrode group.